The present invention relates generally to thin-film magnetic devices. In particular, the present invention is a method for manufacturing thin-film magnetic markers of the type used in electronic article surveillance systems.
Magnetic-type electronic article surveillance (EAS) systems are commonly used to prevent the theft or other unauthorized removal of articles such as books and clothing from libraries, bookstores and department stores. EAS systems of this type include markers which are affixed to the articles to be protected, and an interrogation system including detection panels positioned on opposite sides of a detection corridor leading to exits of the buildings in which the articles are located. The interrogation system produces an alternating magnetic interrogation signal at a predetermined frequency which is transmitted between the detection panels. The markers are relatively small members that include magnetic material. Unless effectively disabled, e.g., at a check-out counter, the magnetic material in the marker emits harmonics of the predetermined interrogation frequency when exposed to the interrogation signal. The interrogation system monitors the presence of these harmonics, and generates an alarm signal when the harmonics are detected. Markers of this type that can be reversibly deactivated are known as dual status markers. EAS systems of this type are described generally in the Elder et al. U.S. Pat. No. 3,665,449 and commercially available from Minnesota Mining and Manufacturing Company (3M).
Markers used in EAS systems of this type typically include elongated strips (e.g., 15.0 cm.times.0.6 cm) of amorphous or polycrystalline ferromagnetic materials exhibiting magnetic properties including those known as low coercivity and high permeability. The Piotrowski et al. U.S. Pat. No. 5,083,112 discloses a more compact marker which includes a plurality of thin-film layers of permalloy, an alloy of nickel and iron (NiFe). The thin-film NiFe layers are separated by nonmagnetic thin-film layers of SiO.sub.x which allow magnetostatic coupling between the layers, but inhibit exchange coupling that can increase the coercivity of the marker. Conventional electron beam (E-Beam) deposition processes are used to coat the thin-film layers of magnetic material on a flexible polymer substrate. The nonmagnetic thin-film layers of SiO.sub.x are grown by conventional sublimation deposition processes.
The thin-film layers of magnetic material are coated onto the substrate in the presence of a magnetic field which causes the magnetic film to form in such a manner that a desired axis of magnetization, known as the "easy" axis of magnetization, is developed in the film. When exposed to an alternating interrogation signal aligned with the easy axis of magnetization, the magnetic states of the thin-film layers are relatively easily driven to saturation, which induces the greatest response in the interrogation signal. In other words, the marker can be easily magnetized and will provide the greatest response in the interrogation signal when it is exposed to the interrogation signal with the easy axis of magnetization aligned with the interrogation signal. In contrast, when the interrogation signal is aligned with the "hard" axis of magnetization, an axis generally perpendicular to the easy axis, the layers become magnetized to only a relatively small value and induce little response to the interrogation signal.
The ability of magnetic-type EAS systems to detect marked articles is therefore dependent to some extent on the orientation of the markers as they are transported through the detection panels. EAS systems typically include interrogation systems that generate an interrogation signal along only one axis. Articles protected by systems of these types will produce the greatest response and be most easily detected when the marker attached to the article has its easy axis of magnetization oriented parallel to the interrogation signal axis as it passes through the detection panels. The magnitude of the induced response, and therefore the ability of the interrogation system to detect the presence of marked articles, decreases with increasing angle between the marker's easy axis of magnetization and the axis of the interrogation signal. The interrogation system's ability to detect the markers is lowest when the easy axis of magnetization is oriented perpendicular to the axis of the interrogation signal.
The Piotrowski et al. U.S. Patent discloses a dual axis or bi-directional magnetic marker which includes a plurality of magnetic thin-film NiFe layers separated by nonmagnetic thin-film SiO.sub.x layers. The dual-axis response is obtained by assembling two magnetic thin-films of the type described above so the easy axes of magnetization of the films are oriented at a perpendicular or 90.degree. angle with respect to one another. Dual axis markers of this type offer considerable advantages over single axis markers since the interrogation system's ability to detect the markers is less sensitive to the orientation of the marker as it is transported through the detection panels. Unfortunately, current methods of manufacturing dual axis markers of this type are relatively complicated and add to the cost of the markers. Because the easy axis of magnetization of the thin-film elements aligns parallel to the axis of the magnetic field applied during deposition, the dual axis markers must be made either by laminating together two or more thin-film elements so their easy axes of magnetization are perpendicular to one another, or depositing the thin-film layers in a coating system capable of producing the orienting magnetic fields in two perpendicular directions. Again, these techniques are relatively complicated and add to the cost of the markers. There is, therefore, a continuing need for more efficient methods for manufacturing dual axis magnetic thin-film markers.